What fact about atomic nuclei suggests the existence of the neutron?
The atom has protons in it. They are positively charged and attract and hold the electrons. But the protons are snuggled in the nucleus, and they don’t like each other. Like charges repel each other. There is a moderating force present that “makes” the protons stick together in there. That’s the fact that suffests something else is at work. And it is the neutron that is the nuclear building block that makes it all stick together.
